Currently, single-layer device efficiency of an organic bulk heterojunction solar cell has almost reached a level of 10% for commercial use. The efficiency is achieved mainly thanks to introduction of a bulk heterojunction device structure. A bulk heterojunction structure is essentially an interpenetrating network structure, and this structure facilitates electron transfer between a donor material and a receptor material.
Currently, a receptor material used in the bulk heterojunction solar cell is usually a fullerene derivative material. The fullerene derivative material has excellent charge transport performance, and matches most donor materials well in terms of an energy level structure. In addition, a fullerene has an advantage of being isotropic in charge transport thanks to a spherical structure of the fullerene, thereby further facilitating charge transport. However, a fullerene receptor material is prone to gather during preparation of an active layer, and therefore, an effective area for charge transport of the fullerene receptor material decreases. In addition, because material preparation and purification are difficult, it is difficult for the fullerene receptor material to be commercially applied.
To find another receptor material to replace the fullerene material, researchers have reported a large quantity of receptor materials. However, these materials cannot be widely applied due to problems of charge transport performance, solubility, and the like. Carbon materials such as graphene and a carbon nanotube are comparable to the fullerene in terms of properties such as a charge transport capability and stability, and therefore, attract attention of the industry. However, because relatively low energy levels of the graphene and the carbon nanotube cannot match an energy level of the donor material, a solar device prepared by using the graphene or the carbon nanotube has a very low open-circuit voltage, and a cell is impractical.